1. Field of the Invention
This invention relates to a automotive radar device and particularly to a automotive radar device that can detect dirt adhering to the surface of a vehicle-side radome on the basis of the signal level of a reflected wave.
2. Description of the Related Art
As a conventional technique of determining dirt adhering to the surface of a vehicle-side radome installed on a vehicle on the basis of the signal level of a reflected wave from the surface of the vehicle-side radome, a process of extracting a low-frequency component of a beat signal by a low-pass filter, A/D-converting the extracted low-frequency component by an A/D converter, performing fast Fourier transform thereto by a fast Fourier transformer, comparing stored frequency spectrum data of a low-frequency component in a state where no dirt is adhering to the radome with frequency spectrum data of an inputted low-frequency component, and detecting that dirt is adhering to the surface of the vehicle-side radome, is performed in a frequency-modulation radar (FM radar) device, as disclosed in Patent Reference 1.
Patent Reference 1: JP-A-10-282229
Generally, a radio-type radar has an advantage that it can be installed in a vehicle-side radome and does not spoil the vehicle design because of the characteristic of its medium, compared with an optical radar.
Therefore, in many cases, a vehicle-side radome is arranged obliquely in front of a radar so as not to spoil the vehicle design. In such cases, a reflected wave from dirt including snow adhering to the surface of the vehicle-side radome is reflected with its reflection angle being equal to its incident angle. Therefore, depending on the angle of the vehicle-side radome, the reflected wave cannot be received and the adhering matter may not be detected.
As reflected components from a very short distance other than the reflection from the adhering matter on the radar radome or the vehicle-side radome, there are leakage between a transmitter and a receiver within a high-frequency circuit, leakage between a transmitting antenna and a receiving antenna and the like. Since the level of these components is decided in accordance with the relation between the phases of transmission wave components (LO waves) to be mixed, the level varies depending on the difference between individual devices and the temperature.
Although there is a technique of, for example, providing a map of judgment reference values in consideration of temperature characteristics, it requires adjustment for each case, leading to increase in the adjustment cost.
Alternatively, there is a technique of constantly learning the level of reflected components from a very short distance and judging the existence/non-existence of adhering matter on the basis of the quantity of change from the learned value.
However, for example, in a case where dirt adheres to the surface of the vehicle-side radome while it is parked at night and then the engine is started the next morning, the learning is started in a state where dirt is already on the surface. Therefore, the quantity of change cannot be detected and the existence of the adhering matter cannot be judged.